linux-kernel - Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo` type

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Message-ID: <aE2sjA4DxFndTZYk@Mac.home>
Date: Sat, 14 Jun 2025 10:08:28 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Benno Lossin <lossin@...nel.org>,
	John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Timur Tabi <ttabi@...dia.com>, Alistair Popple <apopple@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo`
 type

On Fri, Jun 13, 2025 at 11:16:10PM +0900, Alexandre Courbot wrote:
[...]
> >> +                /// Aligns `self` down to `alignment`.
> >> +                ///
> >> +                /// # Examples
> >> +                ///
> >> +                /// ```
> >> +                /// use kernel::num::PowerOfTwo;
> >> +                ///
> >> +                /// assert_eq!(PowerOfTwo::<u32>::new(0x1000).align_down(0x4fff), 0x4000);
> >> +                /// ```
> >> +                #[inline(always)]
> >> +                pub const fn align_down(self, value: $t) -> $t {
> >
> > I'm late to party, but could we instead implement:
> >
> >     pub const fn round_down<i32>(value: i32, shift: i32) -> i32 {
> >         value & !((1 << shift) - 1)
> >     }
> >
> >     pub const fn round_up<i32>(value: i32, shift: i32) -> i32 {
> >         let mask = (1 << shift) - 1;
> >         value.wrapping_add(mask) & !mask
> >     }
> >
> > ? It's much harder to pass an invalid alignment with this.
> 
> It also forces you to think in terms of shifts instead of values - i.e.
> you cannot round to `0x1000` as it commonly done in the kernel, now you

Well, for const values, you can always define:

   const ROUND_SHIFT_0X1000: i32 = 12;

because `0x1000` is just a name ;-)

or we define an Alignment in term of the shift:

    pub struct Alignment {
        shift: i8,
    }

    ipml Alignment {
        pub const new(shift: i8) -> Self {
            Self { shift }
        }
    }

then

    const ALIGN_0x1000: Alignment = Alignment::new(12);

and

    pub const fn round_down_i32(value: i32, align: Alignment) -> i32 {
        ...
    }

My point was that instead of the value itself, we can always use the
shift to represent a power of two, and that would avoid troubles when we
need to check the internal representation.

That said, after some experiments by myself, I haven't found any
significant difference between shift representations vs value
representations. So no strong reason of using a shift representation.

Regards,
Boqun

> need to do some mental gymnastics to know it is actually a shift of `12`.
> Being able to use the actual value to round to is more familiar (and
> natural) to me.